Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene," RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage (&gt;100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. In a basic two-layer EL device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. The extremely thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. Since light emission is directly related to current density through the organic EL medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable fight emission levels (e.g. brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
Further improvement in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos: 4,356,429; 4,539,507; 4,720,432; 4,885,211; 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 4,950,950; 4,769,292, 5,104,740; 5,227,252; 5,256,945; 5,069,975, and 5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; 5,458,977.
For the production of full-color EL display panel, it is necessary to have efficient red, green and blue (RGB) EL materials. With these primary materials, an appropriate combination of their emissions will generate any desirable EL hues or colors, including white. Especially important is the production of blue EL materials, because, given an efficient blue EL material, it is possible to produce other EL colors by a downhill energy transfer process. For instance, a green EL emission can be obtained by doping into a host blue EL material with a small amount of a green fluorescent sensitizing dye. This host-guest energy transfer scheme has been discussed in detail by Tang et al [U.S. Pat. No. 4,769,292]. Similarly, a red EL color can be produced by doping the blue EL host material with a red fluorescent dye. In a somewhat analogous scheme, the fluorescent sensitizing dye may be placed outside the blue EL emitter to effect a shift in the EL emission wavelengths, as discussed by Imai [U.S. Pat. No. 5,015,999]. In this scheme, the sensitizing medium absorbs the blue photon generated by the blue EL emitter, which then emits at longer wavelengths.
In the present invention, it has been found that a class of novel organic materials known as benzazoles is capable of producing highly efficient blue electroluminescence. These materials can be used to produce EL devices with a wide range of visible colors.